Coupling assembly for coupling a rod to a bone anchoring element, and polyaxial bone anchoring device

ABSTRACT

A coupling assembly for coupling a rod to a bone anchoring element, including a receiving part having first and second ends, a recess at the first end for the rod, and an accommodation space for a head of the bone anchoring element, the accommodation space having an opening at the second end to permit insertion of the head into the receiving part, a retainer element configured to be arranged at least partially in the accommodation space and to hold at least part of the head, and a spring element configured to be arranged at least partially in the accommodation space and to be compressed in an axial direction. The retainer element and the spring element are separate parts. When the retainer element and the spring element are in the accommodation space of the receiving part in a first position, the spring element extends into the recess of the receiving part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/596,169, filed Jan. 13, 2015, which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/926,683, filed Jan. 13, 2014,the contents of which are hereby incorporated by reference in theirentirety, and claims priority from European Patent Application EP14151009.9, filed Jan. 13, 2014, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND Field

The invention relates to a coupling assembly for coupling a rod to abone anchoring element and to a polyaxial bone anchoring device withsuch a coupling assembly. The coupling assembly includes a receivingpart with a channel for receiving the rod, an accommodation space foraccommodating a head of a bone anchoring element and a retainer elementfor retaining the head of the bond anchoring element in the receivingpart. Furthermore, a spring element is provided that is biased in such amanner that the retainer element snaps automatically onto the head ofthe bone anchoring element when the head is being inserted.

Description of Related Art

From US 2013/0150852 A1, a polyaxial bone anchor including a housing, abone screw, and a retainer for pivotably coupling the head of the bonescrew to the housing is known. The retainer is positioned into the boreof the housing and includes a plurality of alternating tabs and slotscircumferentially arranged to define a cavity for receiving the headportion of the bone screw therein. The bone anchor further includes aresilient spring means biasing the retainer towards the lower end of thehousing. The head portion of the bone screw may apply a force againstthe retainer opposing and overcoming the biasing force of the resilientspring means. The resilient spring means may be, for example, a wavewasher, a helical spring, an elastomeric member etc. or may becircumferential or helical slots formed in the retainer.

U.S. Pat. No. 8,075,603 B2 describes a fastening system consisting of apolyaxial ball and socket joint used in conjunction with a bone screwhaving threads on one end and a spherical connector on the other endoperating as a pivot point about which a connection assembly moves in apolyaxial fashion. A substantially U-shaped connecting assembly has alower receptacle that operates as a socket for housing an upper retainerring and a lower split retainer ring. The socket is receptive to thespherical connector which is inserted through the lower split retainerring causing a momentary displacement thereof which allows for thepositioning of the spherical connector between the upper and lowerretainer rings. A resilient component, such as two helical springs,positioned between the upper retainer ring and the connecting assemblypermits relative predetermined placement and retention of the sphericalconnector relative to the connector assembly.

SUMMARY

The above polyaxial bone anchors allow for inserting a spherical head ofa bone screw into a receiver by pushing the head against a spring forceof a resilient member. However, there is still a need for a couplingassembly and a polyaxial bone anchor with such a coupling assembly thatis improved with regard to several aspects, such as the efficiency andsafety of the coupling.

It is an object of the invention to provide a coupling assembly forcoupling a rod to a bone anchoring element, and a polyaxial bone anchorcomprising such a coupling assembly, that provides a safe connection ofthe bone anchoring element to the coupling assembly with a low insertionforce, while also providing a high retention force, and where only asmall amount of axial travel or displacement is needed to insert thebone anchoring element into the coupling assembly. Also, the couplingassembly can be easier to manufacture.

The coupling assembly includes a receiving part with an accommodationspace for accommodating the head of the bone anchoring element and aretainer element configured to be positioned at least partially in theaccommodation space. Further, the coupling assembly includes a springelement in the form of, for example, a compression spring that iscompressible in an axial direction and that fully extends around thecentral axis in a circumferential direction and that has an axial lengthso that it can be engaged by an inserted rod. Preferably, the springelement is a wave spring element, which can generate a higher springforce on a given axial length compared to other spring elements.Therefore, the snap-over of the retainer element on the head of the boneanchoring element is facilitated.

The retainer element further may have at least one horizontal slit atits bottom end that may only need a low insertion force to insert thehead into the receiving part. Simultaneously, a retention force thatholds the head in the receiving part is high compared to the insertionforce. Therefore, the bone anchoring element is effectively preventedfrom being pulled-out from the lower opening. In addition, because ofthe small or short insertion path, occurrences of milling under the heador sticking out of the head from the bone can be avoided or reduced.

The coupling assembly may further include a pressure element forexerting pressure onto the head of the bone anchoring element to lockthe bone anchoring element in a specific angular position relative tothe receiving part. The retainer element may encompass at least aportion of the pressure element from an outer side thereof, so thatincreasing a height of the receiving part for accommodating both theretainer element and the pressure element may not be necessary. Hence, alow profile implant can be provided.

In addition, the receiving part is monolithic and sized such that theretainer element and the spring element, as well as the pressureelement, can be mounted from the top opening thereof.

The pressure element may be held in a position such that the head of thebone anchoring element is held by a frictional force exerted by thepressure element onto the head. The frictional force may be such thatthe head can still be pivoted by applying a force to overcome thefrictional force.

The coupling assembly can be assembled in situ with a bone anchoringelement that has been already inserted into a bone or a vertebra.

The head of the bone anchoring element may have at its free end anengagement recess for a driver that comprises a groove shaped andarranged in a spiral-like manner. This allows for transmission of hightorques onto the bone anchoring element. Furthermore, the free endsurface of the anchoring element that has the drive recess may have aspherical shape. In an embodiment where the rod presses directly ontothe head without using a pressure element, the spherical shape ensuresthat a necessary or sufficient contact area between the rod and the headis provided, even at large pivot angles of the bone anchoring element.

With a coupling assembly according to embodiments of the invention, amodular polyaxial bone anchoring device can be provided that may includeseveral bone anchoring elements that differ with respect to the lengthof the shank, anchoring features of the shank, such as different threadtypes and/or thread pitches, different diameters of the shank, and/orwith respect to the shank being cannulated or non-cannulated, amongother features.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become moreapparent from the description of various embodiments using theaccompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of a first embodiment of a boneanchoring device.

FIG. 2 shows a perspective view of the bone anchoring device of FIG. 1in an assembled state.

FIG. 3 shows a perspective view from above of a receiving part accordingto the first embodiment.

FIG. 4 shows a perspective view from the bottom of a receiving partshown in FIG. 3.

FIG. 5 shows a top view of the receiving part shown in FIGS. 3 and 4.

FIG. 6 shows a cross-sectional view of the receiving part shown in FIGS.3 to 5, the cross-section being taken along line A-A in FIG. 5.

FIG. 7 shows a perspective view from above of a retainer elementaccording to the first embodiment.

FIG. 8 shows a perspective view from a bottom of the retainer element ofFIG. 7.

FIG. 9 shows a top view of the retainer element of FIGS. 7 and 8.

FIG. 10 shows a cross-sectional view of the retainer element of FIGS. 7to 9, the cross-section being taken along line B-B in FIG. 9.

FIG. 11 shows a perspective view from above of a spring elementaccording to the first embodiment.

FIG. 12 shows a side view of the spring element shown in FIG. 11.

FIG. 13 shows a top view of the spring element of FIGS. 11 and 12.

FIGS. 14 and 15 show cross-sectional views illustrating steps ofmounting the retainer element to the receiving part according to thefirst embodiment.

FIG. 16 shows a cross-sectional view of a fully assembled couplingassembly according to the first embodiment, with the retainer elementand the spring element, the cross-section being taken in a directiontransverse to an axis of a channel for the rod.

FIGS. 17 to 18 show cross-sectional views illustrating steps of mountingthe coupling assembly of the first embodiment to a bone anchoringelement.

FIG. 19 shows a cross-sectional view of a fully assembled polyaxial boneanchoring device with the coupling assembly according to the firstembodiment, with the inserted bone anchoring element and rod, thecross-section being taken in a plane transverse to an axis of the rod.

FIG. 20 shows a perspective exploded view of a second embodiment of thepolyaxial bone anchoring device, with a second embodiment of thecoupling assembly.

FIG. 21 shows a perspective view from above of a bone anchoring elementaccording to the second embodiment.

FIG. 22 shows a cross-sectional view of the second embodiment of thepolyaxial bone anchoring device, wherein the bone anchoring element isinserted into the coupling assembly, the cross-section being taken in aplane perpendicular to a rod channel axis.

FIG. 23 shows a cross-sectional view of a fully assembled polyaxial boneanchoring device according to the second embodiment, with an insertedand fixed rod.

FIG. 24 shows a perspective exploded view of a third embodiment of thepolyaxial bone anchoring device, with a third embodiment of the couplingassembly.

FIG. 25 shows a cross-sectional view of a receiving part according tothe third embodiment, the cross-section being taken in a planeperpendicular to a rod channel axis.

FIG. 26 shows a perspective view from above of a retainer elementaccording to the third embodiment.

FIG. 27 shows a cross-sectional view of the retainer element shown inFIG. 26.

FIG. 28 shows a perspective view from a top of a spring elementaccording to the third embodiment.

FIG. 29 shows a side view of the spring element of FIG. 28.

FIG. 30 shows a perspective view from above of a pressure elementaccording to the third embodiment.

FIG. 31 shows a perspective view from a bottom of the pressure elementshown in FIG. 30.

FIG. 32 shows a top view of the pressure element of FIGS. 30 and 31.

FIG. 33 shows a cross-sectional view of the pressure element shown inFIGS. 30 to 32, the cross-section being taken along line D-D in FIG. 32.

FIGS. 34 to 35 show cross-sectional views illustrating steps ofassembling the coupling assembly according to the third embodiment.

FIG. 36 shows a cross-sectional view of a step of mounting the couplingassembly of the third embodiment to a bone anchoring element.

FIG. 37 shows a cross-sectional view of a fully assembled polyaxial boneanchoring device according to the third embodiment.

FIG. 38 shows a cross-sectional view of the polyaxial bone anchoringdevice of FIG. 37 with an inserted and fixed rod, wherein thecross-section is taken in a plane transverse to a rod axis.

FIG. 39 shows a cross-sectional view of the polyaxial bone anchoringdevice according to the third embodiment, wherein the cross-section istaken in a plane containing the rod axis.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a bone anchoring device according to a firstembodiment includes a bone anchoring element 1 in the form of a bonescrew having a threaded shank 2 and a head 3. The head 3 has a sphericalsegment-shaped outer surface portion 3 a, including a greatest outerdiameter E of the sphere, and a free end with a recess 3 b forengagement with a screwing-in tool. The bone anchoring device furtherincludes a coupling assembly 4 for coupling a stabilization rod 100 tothe bone anchoring element 1. The coupling assembly 4 includes areceiving part 5, and a retainer element 6 and a spring element 7configured to be arranged in the receiving part 5. A pin 8 may beprovided for securing the retainer element 6 against rotation in thereceiving part 5.

In addition, a locking element 9 in the form of an inner screw isprovided for securing the rod 100 in the receiving part 5 and forlocking the whole device.

Referring in particular to FIGS. 3 to 6, the receiving part 5 is amonolithic part that is substantially cylindrical and has a first end ortop end 5 a, a second end or bottom end 5 b, and a central axis ofsymmetry C passing through the top end 5 a and the bottom end 5 b. Abore 51 is provided that is coaxial with the central axis C. In a firstregion adjacent to the top end 5 a, the receiving part 5 has asubstantially U-shaped recess 52 with a bottom directed towards thebottom end 5 b, and two free lateral legs 52 a, 52 b extending towardsthe top end 5 a. On the legs 52 a, 52 b, an internal thread 53 isprovided that cooperates with the locking element 9. The channel formedby the U-shaped recess 52 is sized so as to receive the rod 100 thereinfor connecting two or more bone anchoring devices. In a region thatextends from a distance above (i.e., closer to the top end 5 a than) thebottom of the U-shaped recess 52 to a distance from the bottom end 5 b,the bore 51 has a greater diameter compared to other portions of thebore 51 so that an accommodation space 54 is formed to receive the head3 of the bone anchoring element 1 and to receive the retainer element 6as well as the spring element 7. The accommodation space 54 has, at itslower end adjacent to the bottom end 5 b of the receiving part, a seatportion 54 a for the retainer element 6. The seat portion 54 a has asmaller diameter than the main portion of the accommodation space 54 andconically tapers toward the bottom end 5 b of the receiving part 5.Between the seat portion 54 a and the rest of the accommodation space54, a shoulder 54 b is provided that may also function as a stop for theretainer element 6. An upper edge 54 c between the accommodation space54 and the rest of the bore 51 forms a stop for the spring element 7.

The accommodation space 54 further has an opening 55 at the bottom end 5b, the inner diameter of which is larger than the greatest outerdiameter E of the head 3 of the bone anchoring element 1, so that thehead 3 can be inserted from the bottom end 5 b. The edge or perimetersurrounding the opening 55 has a through-hole 56 that extends into theaccommodation space 54 and that serves as a press-fit accommodation ofthe pin 8. An axis of the through-hole 56 may be parallel to the centralaxis. The through-hole 56 of the receiving part 5 is positioned in acircumferential direction approximately corresponding to a center of oneof the legs 52 a, 52 b.

Referring more in detail to FIGS. 7 to 10, the retainer element 6 willbe explained. The retainer element 6 resembles a cap-like part, andincludes a first end or top end 6 a and an opposite second end or bottomend 6 b. Adjacent to the bottom end 6 b, a spherical segment-shapedrecess 61 extends into the retainer element 6 with an inner diameterthat matches an outer diameter of the spherical surface portion 3 a ofthe head 3. The extension of the recess 61 in an axial direction is suchthat when the head 3 is inserted into the recess 61, the bottom end 6 bextends below (i.e., closer to the shank 2 than) the region of the head3 with the greatest diameter E.

A coaxial bore 62 extends from the top end 6 a into the recess 61 toallow access to the head 3 of the anchoring element 1 with a tool. Anouter surface of the retainer element 6 has a first portion 63 adjacentto the top end 6 a that is substantially conically shaped and thattapers and narrows towards the first end 6 a. The first portion 63 mayhave two opposite flattened sides 63 a that facilitate gripping theretainer element 6 with a tool (now shown) for assembling the retainerelement 6 and the receiving part 5.

Approximately in the region with the largest inner diameter of therecess 61, an outwardly protruding annular edge 64 is formed with amaximum outer diameter that is only slightly smaller than an innerdiameter of the accommodation section 54 of the receiving part 5, asshown, for example, in FIG. 16. The outwardly protruding annular edge 64has an upper side 64 a that is configured to support the spring element7. Flattened portions 64 b of the annular edge 64 are provided andarranged at substantially 90° with respect to the flattened portions 63a of the first portion 63. On one side at substantially 90° with respectto the flattened portions 64 b in a circumferential direction, asubstantially U-shaped recess 65 is provided in the outer surface of theannular edge 64 which extends in an axial direction. The recess 65 isconfigured to receive the pin 8 therein.

Adjacent to the bottom end 6 b, the retainer element 6 has the form of aslit ring 66. The slit ring 66 has a substantially conical outer shapethat matches the inner shape of the seat portion 54 a of theaccommodation space 54. The inner surface of the slit ring 66 forms aportion of the spherical recess 61, so that the inner surface of theslit ring 66 generates a seat for the head 3, to provide a ball andsocket joint between the receiving part 5 and the bone anchoring element1 when the retainer element 6 is mounted to the receiving part 5. Asdepicted in FIG. 8, the slit ring 66 is formed by a first vertical slit66 b extending from the bottom end 6 b in a substantially verticaldirection. From the vertical slit 66 b two opposite horizontal slits 66c, 66 d extend circumferentially around the central axis C. Thehorizontal slits 66 c, 66 d end in widened portions 66 e, 66 f. Betweenthe end portions 66 e, 66 f a connecting portion 66 g is formed thatconnects the slit ring 66 to the remainder of the retainer element 6.Hence, the retainer element 6 is a monolithic part including the slitring 66. The width of the vertical slit 66 b and of the horizontal slits66 c, 66 d as well as the width of the connecting portion 66 g, may beselected such that a desired flexibility of the slit ring 66 isobtained. By means of the slit ring 66, the retainer element 6 isconfigured to be expanded and compressed in a radial direction. As canbe seen in particular in FIGS. 8 and 10, the vertical slit is positionedcircumferentially at substantially the same position as the U-shapedrecess 65, while the connecting portion 66 g is at a circumferentiallyopposite position.

As illustrated in FIG. 16, a total axial length of the retainer element6 when the retainer element 6 is inserted into the receiving part 5 andwhen the slit ring 66 is seated in the seat portion 54 a of thereceiving part 5 is such that the top end 6 a of the retainer element 6is approximately at or slightly above the axial height of the bottom ofthe U-shaped recess 52. It shall be noted that the retainer element 6acts also as a pressure element on an inserted head 3, as it not onlyprevents removal of the head 3, but also exerts pressure to the head 3from the side and from above.

Referring now to FIGS. 11 to 13, the spring element 7 is formed as awave spring, and includes substantially circular turns each made out ofa flat strip, such as a flat wire. A cross-section of the flat strip maybe rectangular. In the embodiment shown, the wave spring has four turns71, 72, 73, 74. However, in other embodiments, a plurality of turns moreor less than four turns may be provided. Each turn has a plurality ofwave crests 70 a and a plurality of wave troughs 70 b that arealternately arranged and that form a ring. The turns 71, 72, 73, 74 thatare consecutively stacked in an axial direction around the central axisC are connected to each other, such that adjacent turns offset from oneanother in a circumferential direction by the length of a crest portion(or a trough portion), as can be seen in particular in FIGS. 11 and 12.This specific shape is known as a crest-to-crest wave spring.

The turns of the spring element 7 may each be made out of separate flatstrips that may be connected to each other, for example, by welding.Alternatively, the spring element can be made out of a single continuousflat strip that is wound around a central axis, so as to form a wavespring shape. Many modifications are possible. The number of turns, i.e.the axial length of the spring element 7, and various other springproperties can be selected such that a desired compression force andreturn force can be provided by the spring element 7. The axial lengthof the spring element is selected such that, as can be seen in FIG. 17to 19, the spring element 7 can be positioned onto the first portion 63of the retainer element 6 and fills the accommodation space 54 betweenthe upper surface 64 a of the annular edge 64 of the retainer element 6and the upper edge 54 c of the accommodation space 54. An outer diameterof the spring element 7 is slightly smaller than an inner diameter ofthe accommodation space 54.

When the spring element 7 is in the accommodation space 54, the springelement 7 may be in a biased condition so that it exerts a pre-load ontothe retainer element 6, but can still be compressed further.

An advantage of a wave spring compared to a helical compression springis that a wave spring produces the same or similar compression andreturn force, but can have a shorter axial length than a helicalcompression spring. As a consequence thereof, the necessary space forthe spring element 7 may be smaller or reduced, and/or the travel pathfor inserting the anchoring element against the spring force can beminimized or reduced.

The bone anchoring device, as a whole or in part, may be made of one ormore bio-compatible materials, such as a bio-compatible metal or a metalalloy, for example titanium, stainless steel, a nickel-titanium alloy,for example nitinol, or of bio-compatible plastic materials, such as,for example, polyetheretherketon (PEEK), or of a bio-compatible ceramicmaterial. In particular, it may be contemplated that the spring element7 is made of a superelastic nickel-titanium alloy or of beta titanium.

The assembly of the coupling assembly will be explained with referenceto FIGS. 14 to 16. First, the pin 8 can be inserted beforehand in thethrough-hole 56 of the receiving part 5. Next, the retainer element 6 isinserted into the receiving part 5 from the top end 5 a. The retainerelement 6 can be inserted in a 90° tilted manner such that its centralaxis is oriented at 90° with respect to the central axis C of thereceiving part. A less than 90° tilted orientation can also be used forinsertion, as illustrated. Then, when a portion of the bottom end 6 band the annular edge 64 enters the accommodation space 54, the retainerelement 6 can be tilted again, so that its central axis becomes coaxialwith the central axis C of the receiving part 5. The orientation of theretaining element is such that, as depicted in FIG. 15, the U-shapedaxial recess 65 provided at the annular edge 64 engages the pin 8.

When the retainer element 6 is seated with the slit ring 66 in the seatportion 54 a of the receiving part 5, the spring element 7 is insertedinto the accommodation space 54 such that the spring element 7 encirclesthe upper portion 63 of the retainer element 6 and rests on the upperside 64 a of the annular edge 64. The top end 7 a of the spring element7 abuts against the stop 54 c provided at the upper side of theaccommodating space 54. In this condition, the spring element 7 issomewhat compressed, so that the spring element 7 exerts a biasing forceonto the retainer element 6 to hold the slit ring 66 in the seat 54 a.When the slit ring 66 is placed in to the seat 54 a, the lower edge 6 bof the retainer element 6 can project slightly out of the lower opening55 of the receiving part 5. A portion of the spring element 7 extendsabove the bottom of the channel 52 of the receiving part 5, so that thespring element 7 can be engaged by the rod 100.

Next, as depicted in FIGS. 17 and 18, the head 3 of the bone anchoringelement 1 is inserted through the lower opening 55 into the receivingpart 5. As shown in FIG. 18, when the head 3 enters the accommodationspace 54 through the lower opening 55, the slit ring 66 is pushedupwards out of the seat 54 a. Simultaneously, the spring element 7 iscompressed by the upward movement of the retainer element 6. Inaddition, the slit ring 66 is expanded when the head 3 further entersinto it. The accommodation space 54 provides space for the expansionafter the slit ring 66 is pushed out of the seat 54 a. When thecounterforce exerted by the compressed spring element 7 is greater thanthe necessary force for expanding the slit ring 66 and for sliding theslit ring 66 over the portion of the head 3 having the largest diameterE, the spring force of the compressed spring element 7 causes the slitring 66 to snap over the head 3, so that the lower edge 6 b slides overthe region of the head 3 with the largest diameter E. The head 3 can beinserted into the retainer element 6 only to such an extent that thehead 3 abuts against the upper portion of the spherical recess 61 of theretainer element 6. The retainer element 6 and the spring element 7cannot escape through the top end 5 a of the receiving part 5 because ofthe stop 54 c provided in the accommodation space 54.

When the slit ring 66 is positioned below the portion of the head 3 withthe greatest diameter E, the head 3 can no longer be pulled-out throughthe lower opening 55.

The slit ring 66 may have a slight undersize with respect to the size ofthe head 3 when the slit ring 66 is unbiased, such that when the slitring 66 encompasses the head 3, a frictional force is exerted onto thehead 3. Hence the head 3 may be held by friction within the receivingpart 5, and the receiving part 5 can be maintained or held temporarilyat a specific angular position with respect to the bone anchoringelement 1 before locking of the bone anchoring element 1 relative to thereceiving part 5. The spring force exerted by the biased spring element7 may also contribute to the friction hold of the head 3 in thereceiving part 5.

As depicted in FIG. 19, in a next step, the rod 100 is inserted into theU-shaped recess 52 and the locking element 9 is inserted in thereceiving part 5. When the locking element 9 is tightened, the rod 100presses onto the top end 6 a of the retaining element 6 and alsocontacts the spring element 7. Final tightening of the locking element 9locks the slit ring 66 of the retaining element 6 and the head 3 in thereceiving part 5. As can be seen in FIG. 19, in the locked condition,the rod 100 also compresses the spring element 7.

In use, the bone anchoring element 1 may be inserted into the bone orinto a vertebra prior to mounting the coupling assembly 4. In analternative manner of use, the bone anchoring element 1 and the couplingassembly 4 are pre-assembled and thereafter inserted into the bone. Aplurality of bone anchoring devices can be connected through astabilization rod 100.

Referring to FIGS. 20 and 21, a second embodiment of a polyaxial boneanchoring device will be described. The polyaxial bone anchoring deviceaccording to the second embodiment differs from the polyaxial boneanchoring device according to the first embodiment in the design of thecoupling assembly, and in particular, in the design of the retainerelement. All parts that are similar or identical to the previousembodiments are designated with the same reference numerals, and thedescriptions thereof will not be repeated.

The coupling assembly 4′ includes a retainer element 6′, which lacks thefirst portion 63 of the retainer element 6 of the first embodiment. Theretainer element 6′ includes an annular portion 64′ with a cylindricalinner surface and a slit ring 66 that is identical or similar to theslit ring 66 of the first embodiment. With such a design, the head 3′ ofthe bone anchoring element 1′ can protrude out of the top end 6 a of theretainer element 6′. The size of the retainer element 6′ is such thatthe head 3′ protrudes out of the retainer element 6′ to an extent thatallows the rod 100 to press directly onto the head 3′.

The bone anchoring element 1′ according to the second embodimentincludes a spherical head 3′ that also has a spherical outer surface ata free upper end. In the embodiment, the head as a whole issubstantially spherical. The recess 3 b′ for the tool may have wingsthat extend in a spiral-like manner from a center point of the upperfree end. In the embodiment shown, the recess 3 b′ includes four wingsthat are formed by a groove 31 that forms the outer contour of a crosswith arms each bent in a same direction. By means of this, theengagement surface formed by the recess 3 b′ for a tool or driver isenhanced or more robust compared to usual polygon engagements or otherrecesses or engagements. Therefore, the loads that can be transferredonto the head 3′ are higher. Recesses for drivers and correspondingtools with a similar shape are known, for example, under the trademarkMortorq®. It may be contemplated that similar shapes can be used asdrive recesses for the head 3′.

As depicted in FIG. 22, when the head 3′ has entered the receiving part5 and the retainer element 6′, the upper portion of the head 3′including the drive recess 3 b′ protrudes out from the top end 6 a ofthe retainer element 6′. Also in this embodiment, the head 3 cannot bepulled out through the lower opening 55 once the retainer element 6′ isseated in seat 54 a of the receiving part 5. The spring element 7projects into the channel 52 for the rod 100. Hence, the rod 100 engagesthe spring element 7 when it is inserted into the recess 52. When therod 100 is moved down via the locking element 9, as depicted in FIG. 23,the rod 100 compresses the spring element 7, which in turn urges theretainer element 6′ into the seat 54 a of the receiving part 5. Finally,the rod 100 presses onto the upper surface of the head 3′, whereby thehead 3′ is pressed into the seat provided by the slit ring 66 of theretainer element 6′. Because of the spherical shape of the head 3′,sufficient pressure is exerted onto the head 3′ in both non-pivoted andpivoted states of the bone anchoring element 1′ relative to thereceiving part 5.

The other steps of mounting the retainer element 6′ to the receivingpart 5 are the same as or similar to the first embodiment.

A third embodiment of a polyaxial bone anchoring device including acoupling assembly will be described with reference to FIGS. 24 to 39.First, as can be seen in FIG. 24, the coupling assembly 4″ includes areceiving part 5″, a retainer element 6″, a spring element 7″, andadditionally a pressure element 10 for exerting pressure onto the head 3of the bone anchoring element 1. It shall be noted that in thisembodiment the bone anchoring element 1 is the same as or similar to thebone anchoring element 1 of the first embodiment. However, embodimentsof the invention should not be limited thereto, and another boneanchoring element, for example, that of the second embodiment, can alsobe used. Parts and portions of the third embodiment that are identicalor similar to that of the first or second embodiments are marked withthe same reference numerals, and the descriptions thereof will not berepeated.

The receiving part 5″ of the third embodiment is depicted in across-sectional view taken in a place perpendicular to an axis of therod recess 52 in FIG. 25. The receiving part 5″ lacks the through-hole56 for the pin 8 at the bottom end 5 b. For the purpose of holding apressure element 10 inside the receiving part 5″ and for preventingrotation of the inserted pressure element 10, the receiving part 5″includes two transverse pin holes 57 a, 57 b that are positioned in acircumferential direction substantially at centers of the legs 52 a, 52b. The pin holes 57 a, 57 b are configured to accommodate pins 11 a, 11b in a press-fit manner. The length of the pins 11 a, 11 b is such thatthe pins 11 a, 11 b can protrude into the bore 51 of the receiving part5″ to engage the pressure element 10. The axial position of thethrough-holes 57 a, 57 b is such that, as depicted for example in FIG.36, the pressure element 10 is held by the pins 11 a, 11 b at its upperend, to be described in greater detail below.

The retainer element 6″ differs from the retainer elements of the firstand the second embodiments in that there is a portion 63″ adjacent to anupper surface 64 a of the annular outwardly protruding edge 64″ thatincludes a plurality of circumferentially arranged upstanding slightlyresilient wall portions 63 a that are separated by axial slots so as toprovide elasticity. Each wall portion 63 a has an inwardly protrudingedge 63 b. The inwardly protruding edges 63 b are configured to engage aportion of the pressure element 10 so that the pressure element 10 andthe retainer element 6″ can be coupled together. An inner and an outersurface of the wall portions 63 a is each substantially conical,tapering and narrowing towards the top end 6 a.

The annular edge 64″ lacks the recess of the retainer element forreceiving the pin in the first embodiment, as there is no pin in thethird embodiment for holding the retainer element. The slit ring 66 isthe same as or similar to the slit ring 66 from the first and secondembodiments.

As depicted in FIGS. 28 and 29, the spring element 7″ is also a wavespring, preferably of the type of a crest-to-crest wave spring. Thespring element 7″ has three turns in the embodiment shown, but thenumber of turns is not limited to three, and the spring element in otherembodiments can have two or more than three turns, depending on theavailable space in the accommodation space 54 and on the springcharacteristics of the spring element. An inner and an outer diameter ofthe spring element 7″ is such that the spring element 7″ is configuredto be supported on the upper side 64 a of the annular edge 64″ of theretainer element 6″ and to extend around the first portion 63″ havingthe flexible wall portions 63 a.

The pressure element 10 has a first end or top end 10 a and a second endor bottom end 10 b. Adjacent to the top end 10 a there is asubstantially cylindrical section with a first outer diameter that isonly slightly smaller than the inner diameter of the bore 51 of thereceiving part 5″ so that the first portion 101 can be slidinglyarranged in the bore 51. Adjacent to the top end 10 a there is also atransverse recess 102 with substantially parallel side walls and asubstantially V-shaped bottom 102 a that forms a channel for receivingthe rod 100, where the rod axis is perpendicular to the central axis C.The V-shaped bottom 102 a is configured to support rods of differentdiameter. By the recess 102, two upstanding legs 102 b, 102 c are formedthat are provided at their top side with smaller U-shaped recesses 103a, 103 b, respectively, which are open to the top end 10 a and whichextend transverse to an axis of the channel. The recesses 103 a, 103 bare configured to receive a portion of the pins 11 a, 11 b,respectively, as can be seen in FIGS. 35 and 36, for example. Each ofthe legs 102 b, 102 c, respectively has an outwardly projecting flange104 a, 104 b that is arranged substantially at the height of theV-shaped bottom 102 a and that extends in a circumferential directionaround a portion of each respective leg 102 b, 102 c.

At either end of the bottom of the channel 102 for the rod 100, recesses105 a, 105 b are provided that allow for insertion of the pressureelement 10 when the pins 11 a, 11 b are already mounted into the pinholes 57 a and 57 b of the receiving part 5″.

The pressure element 10 further includes, adjacent to the bottom end 10b, a second portion 106 with a smaller diameter compared to the firstportion 101. The second portion 106 is substantially cylindrical and hasa spherical segment-shaped recess 107 adjacent to the bottom end 10 b.The spherical recess 107 is configured to exert pressure onto the head 3of the bone anchoring element 1. An outwardly protruding edge 108 isprovided at the bottom end 10 b for engaging the inwardly protrudingedges 63 b of the retainer element 6″ to couple the pressure element 10to the retainer element 6″. The edge 108 has a chamfered lower side forfacilitating insertion of the second portion 106 into the retainerelement 6″. Furthermore, the pressure element 10 has a coaxial bore 109for providing access to the head 3 of the bone anchoring element 1 witha tool (not shown).

The assembly of the coupling assembly 4″ will be explained withreference to FIGS. 34 and 35. The retainer element 6″ is inserted fromthe top end 5 a of the receiving part 5″ in a tilted manner. When theretainer element 6″ has reached the accommodation space 54, the retainerelement 6″ is tilted back and placed onto the bottom of theaccommodation space 54, with the slit ring 66 being in the seat 54 a.

When the retainer element 6″ has been inserted into the receiving part5″, the spring element 7″ is also inserted from the top end 5 a of thereceiving part 5″ and placed on top of the retainer element 6″ so thatthe spring element 7″ rests on the annular edge 64″.

Next, the pressure element 10 is inserted into the receiving part 5″from the top end 5 a. As the pins 11 a, 11 b may already be received inthe pin holes 57 a, 57 b of the receiving part 5″, the pressure element10 can be inserted in a 90° rotated position, so that the recess 102 isoriented at 90° with respect to the U-shaped recess 52 of the receivingpart 5″ and the flanges 104 a, 104 b are aligned with the U-shapedrecess 52. In this configuration, the pins 11 a, 11 b can pass throughthe slits 105 a, 105 b. When the pressure element 10 enters the upperportion 63″ of the retainer element 6″, the flexible wall portions 63 aare deflected slightly outward to allow the insertion of the bottom end10 b of the pressure element 10, until the inwardly protruding edge 63 bof the retainer element 6″ snaps behind or over the outward edge 108 ofthe lower portion of the pressure element 10. Once the flanges 104 a,104 b have entered the accommodation space 54, the pressure element 10can be rotated so that the channel 102 of the pressure element 10 andthe U-shaped recess 52 of the receiving part 5″ are aligned. Rotationmay only be possible when the top end 10 a of the pressure element 10 isbelow the pins 11 a, 11 b.

After alignment of the pressure element 10, the pins 11 a, 11 b arereceived in the U-shaped recesses 103 a, 103 b, respectively, of thepressure element 10. The bottom of the U-shaped recesses 103 a, 103 bform an abutment for the pressure element 10 and prevent escaping of thepressure element 10 through the top end 5 a of the receiving part 5″. Inthe preassembled state of the coupling assembly 4″, as depicted in FIG.35, the spring element 7″ is slightly pre-compressed.

The mounting of the coupling assembly 4″ to the bone anchoring element 1is shown in FIGS. 36 and 37. As in the first embodiment, the head 3enters through the lower opening 55 into the retainer element 6″, andtherefore into the accommodation space 54. The slit ring 66 is pushedout of seat 54 a and the retainer element 6″ is moved upward, wherebythe contacting surfaces of the outer rim 108 of the pressure element 10and of the inwardly protruding edges 63 b of the retainer element 6″disengage. During upward movement of the retainer element 6″, the springelement 7″ is compressed, whereby the spring element 7″ abuts againstthe lower side of the flanges 104 a, 104 b and/or other portions of thelower side of the first portion 101 of the pressure element 10. Theupward movement of the pressure element 10 is limited by the pins 11 a,11 b.

During the insertion of the head 3 into the retainer element 6″, theslit ring 66 expands. As soon as the counterforce exerted by thecompressed spring element 7″ is greater than the necessary force forexpanding the slit ring 66 and for sliding the slit ring 66 over theportion of the head 3 with the largest diameter E, the spring force ofthe compressed spring element 7″ causes the slit ring 66 to snap overthe head 3 so that the lower edge 6 b slides over the region with thelargest diameter E. The head 3 can be inserted until it abuts againstthe spherical recess 107 of the pressure element 10. When the springforce shifts the retainer element 6″ downward, the inwardly protrudingedges 63 b and the outward rim 108 engage again.

The pre-stress exerted by the spring element 7″ and/or a slightundersize of the slit ring 66 compared to the size of the head 3 leadsto a frictional hold of the head 3 in the receiving part 5″ beforefinally locking the head 3. Also, when the slit ring 66 enters the seat54 a again, a removal of the head 3 from the receiving part 5″ may nolonger be possible.

Finally, as can be seen in FIGS. 38 and 39, the rod 100 is inserted andpressed down by tightening the locking element 9 in the receiving part5″. The pressure exerted by the pressure element 10 onto the head 3further presses the slit ring 66 into the seat 54 a, and finaltightening locks the head 3 and the retainer element 6″ in the receivingpart 5″.

Further modifications of the disclosed embodiments may be contemplated.For example, for the bone anchoring element, various different kinds ofanchoring elements can be used and combined with the receiving part. Theanchoring elements may be, for example, screws with different lengths,screws with different diameters, cannulated screws, screws withdifferent thread forms, nails, hooks, etc. For some anchoring elements,the head and the shank may also be separate parts that can be connectedto each other.

Some possible modifications of the receiving part may include, forexample, instead of the U-shaped recess being perpendicular to thecentral axis, a recess for the rod may be inclined, open to the side, orin the form of a closed channel. Other kinds of locking devicesincluding outer nuts, outer caps, bayonet locking devices, or others canalso be utilized. In particular, a two-part locking device that includesa first locking element that exerts pressure via the pressure elementonto the head and a second locking element that exerts pressure onlyonto the rod to lock the head and the rod independently may also beused. In some embodiments, the inner surface portion of the pressuremember that contacts the head may not necessarily be spherically-shaped.The inner surface portion may have any other shape that is suitable toexert pressure onto the head. Also, the design of the pressure elementcan be different and is not limited to the specific design shown in thethird embodiment.

Instead of the pin for retaining the pressure element and for aligningthe pressure element with respect to the channel of the receiving partfor receiving the rod, other retaining mechanisms can be used.

In some embodiments, the head of the bone anchoring element is notrotationally symmetric. For example, the head may have two opposite flatsurface portions between two spherically-shaped outer surface portions,to achieve pivoting in only one plane.

Instead of the slit ring, a plurality of vertically extending slits or acombination of substantially vertically and substantially horizontallyextending slits may be provided.

The seat for the slit ring and the outer surface of the slit ring neednot be conical. Any shape that provides for safe holding of the slitring may be contemplated, such as, for example, a spherical shape.

For the spring element, other spring elements can be used. For example,a helical spring encircling the central axis may be used. In addition,other spring elements like elastomeric cushions may be contemplated.

It shall be noted that parts of the different embodiments described mayalso be mixed among each other or exchanged, so that a variety offurther embodiments can be generated.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but is instead intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, and equivalents thereof.

The invention claimed is:
 1. A polyaxial bone anchoring devicecomprising: a bone anchoring element comprising a shank for anchoring toa bone and a head; and a receiving part having a first end, a secondend, a recess at the first end for receiving a rod, and an accommodationspace at the second end for accommodating the head, wherein when thehead is in the accommodation space, an angular position of the headrelative to the receiving part is configured to be locked; wherein thehead of the bone anchoring element forms a sphere extending from theshank to a free end surface on a side of the head opposite the shank,and wherein an engagement recess for engaging a tool is formed on thehead and extends uninterruptedly entirely around a portion of the sphereto define the free end surface and to separate the free end surface fromother portions of the sphere.
 2. The polyaxial bone anchoring device ofclaim 1, wherein the engagement recess has a plurality of wing portionseach extending from a center of the free end surface.
 3. The polyaxialbone anchoring device of claim 2, wherein each of the wing portions ofthe engagement recess extends in a curved manner from the center of thefree end surface.
 4. The polyaxial bone anchoring device of claim 1,wherein the engagement recess is rotationally asymmetric relative to alongitudinal axis of the bone anchoring element.
 5. The polyaxial boneanchoring device of claim 1, wherein a longitudinal axis of the boneanchoring element intersects the free end surface of the head.
 6. Thepolyaxial bone anchoring device of claim 1, wherein at least a portionof the head that is positioned radially outside of the engagement recessis located at an axial position that is farther away from the shank thanat least a portion of the head positioned radially inside the engagementrecess is from the shank.
 7. The polyaxial bone anchoring device ofclaim 1, wherein the engagement recess forms a non-circular profile on aportion of the head that is positioned radially outside of theengagement recess.
 8. The polyaxial bone anchoring device of claim 1,wherein the receiving part defines an opening at the second end, andwherein the head of the bone anchoring element is insertable through theopening into the accommodation space.
 9. A polyaxial bone anchoringdevice comprising: a bone anchoring element comprising a shank foranchoring to a bone and a head; a receiving part having a first end, asecond end, a recess at the first end for receiving a rod, and anaccommodation space at the second end for accommodating the head,wherein when the head is in the accommodation space, an angular positionof the head relative to the receiving part is configured to be locked;and a retainer element positionable at least partially in theaccommodation space and configured to contact the head to lock the head;wherein the head of the bone anchoring element has a rounded free endsurface on a side of the head opposite the shank, an engagement recessfor engaging a tool, wherein the engagement recess extends around therounded free end surface and at least partially defines the rounded freeend surface, and a rounded outer surface having at least a portion thatis spaced apart from the rounded free end surface in an axial directionby the engagement recess and that extends between the engagement recessand the shank to facilitate adjusting of the angular position of thehead relative to the receiving part.
 10. The polyaxial bone anchoringdevice of claim 9, wherein the retainer element is insertable from thefirst end of the receiving part into the accommodation space.
 11. Thepolyaxial bone anchoring device of claim 9, wherein when the retainerelement and the head are in the receiving part and the head is held inthe retainer element, the head extends into the recess of the receivingpart and is configured to contact the rod when the rod is received inthe recess.
 12. The polyaxial bone anchoring device of claim 9, whereina longitudinal axis of the bone anchoring element intersects the roundedfree end surface of the head.
 13. The polyaxial bone anchoring device ofclaim 9, wherein the engagement recess extends uninterruptedly entirelyaround the rounded free end surface of the head to separate the roundedfree end surface from other portions of the head.
 14. The polyaxial boneanchoring device of claim 13, wherein the engagement recess forms anon-circular profile on a portion of the head that is positionedradially outside of the engagement recess.
 15. The polyaxial boneanchoring device of claim 9, wherein the receiving part defines anopening at the second end, and wherein the head of the bone anchoringelement is insertable through the opening into the accommodation space.16. A polyaxial bone anchoring device comprising: a bone anchoringelement comprising a shank for anchoring to a bone, a head, and alongitudinal axis extending through the shank and the head; and areceiving part having a first end, a second end, a recess at the firstend for receiving a rod, and an accommodation space at the second endfor accommodating the head, wherein when the head is in theaccommodation space, an angular position of the head relative to thereceiving part is configured to be locked; wherein the head of the boneanchoring element has a rounded free end surface on a side of the headopposite the shank and an engagement recess for engaging a tool, whereinthe engagement recess extends around the rounded free end surface and atleast partially defines the rounded free end surface, and wherein atleast a portion of the head that is positioned radially outside of theengagement recess is located at an axial position that is farther awayfrom the shank than at least a portion of the rounded free end surfaceis from the shank.
 17. The polyaxial bone anchoring device of claim 16,wherein the longitudinal axis of the bone anchoring element intersectsthe rounded free end surface of the head.
 18. The polyaxial boneanchoring device of claim 16, wherein the engagement recess extendsuninterruptedly entirely around the rounded free end surface of the headand separates the rounded free end surface from other portions of thehead.
 19. The polyaxial bone anchoring device of claim 18, wherein theengagement recess forms a non-circular profile on a portion of the headthat is positioned radially outside of the engagement recess.
 20. Thepolyaxial bone anchoring device of claim 16, wherein the head of thebone anchoring element forms a sphere extending from the shank to therounded free end surface, and wherein the engagement recess is formed onan outer surface of the sphere.